qemuBuildVideoCommandLine: Generate via JSON

qemuBuildVideoCommandLine: Generate via JSON

We control the following properties of the devices in question:

'virtio-gpu'
  virgl=<bool>           - on/off (default: true)

'qxl'
  ram_size=<uint32>      -  (default: 67108864)
  vram_size=<uint64>     -  (default: 67108864)
  vram64_size_mb=<uint32> -  (default: 4294967295)
  vgamem_mb=<uint32>     -  (default: 16)
  max_outputs=<uint16>   -  (default: 0)

'vhost-user-gpu'
  max_outputs=<uint32>   -  (default: 1)
  chardev=<string>

'VGA'
  vgamem_mb=<uint32>     -  (default: 16)

'bochs-display'
  vgamem=<size>          -  (default: 16777216)

common for all devices:
  xres=<uint32>          -  (default: 0)
  yres=<uint32>          -  (default: 0)

The only noticable change is using memory size in bytes for
'bochs-display' instead of kibibytes.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>